1. Field of the Invention
The present invention relates generally to a MOS transistor. More specifically, the invention relates to a MOS transistor which has a pair of electrodes opposing each other across a semiconductor layer. Further particularly, the invention relates to a thin film MOS transistor which has enhanced electron mobility.
2. Description of the Background Art
Conventionally, such types of thin film MOS transistors are provided with pairs of gate electrodes opposing each other across a semiconductor layer. In general, the semiconductor layers formed between the gate electrodes constitute thin film in a range of 3,000 nm to 5,000 nm. An energy level diagram of typical thin film MOS transistor is shown in FIG. 1. As will be seen from the energy level Ec at the end of energy transfer path as illustrated in FIG. 1, the lateral gradient of the transfer band over the energy transfer path tends to become substantial. This is caused by local concentration of the electron path on the surface of a channel. This causes narrowing of the electron path on the channel surface and thus causes scattering of free electrons, lowering electron mobility mobility .mu..
As will be appreciated, the electron mobility .mu. is one of the principle factors for determining the performance of this type of transistor, especially for operational speed of the transistor. On the other hand, the requirement for higher operational speed transistors becomes higher for widening the application of such semiconductor transistors. In this view, the thin film MOS transistors are required higher electron mobility.